1. Field of the Invention
The present invention relates to a medium on which information is recorded by using light and an information recording and reproduction device using the same.
2. Description of the Related Art
Some of the terms used in the following are expressions used for Blu-ray Disc (BD). These might be called by different terms for systems other than BD. However, one of ordinary skill in the art can easily read the terms as the different ones.
The enlargement of the recording capacity of optical discs has been realized by increasing the number of recording layers per disc as well as by shortening the wavelength of the light source and increasing the numerical aperture (NA) of the objective lens. For BD, using a blue semiconductor laser and a high NA objective lens with an NA of 0.85, a recording capacity of 50 GB has been realized with two layers. Further, in 2010, by increasing the number of recording layers to three to four and increasing the surface recording capacity at the same time, BDXL having a recording capacity of not less than 100 GB was put into practical use.
Although the shortening of the wavelength of the recording and reproduction light and the increase of the NA of the objective lens are near the limit, a method explained in Jpn. J. Appl. Phys., Vol 32 (1993) pp. 5324-5328 (Non-Patent Document 1) is considered in which by using the land-groove effect, recording is performed on the land and the groove to narrow the track pitch, thereby further increasing the surface recording capacity. Hereinafter, this method using the land-groove effect will be referred to as land-groove method. Actually, DVD-RAM is known as an optical disc system to which the land-groove method is applied.
When a recordable-type optical disc system using the land-groove method is constructed, a method of forming the address information on the disc is one of the fundamental problems to be solved. On many recordable-type optical discs, information is recorded along a guide groove formed spirally on the disc substrate, or the land. By wobbling the groove at predetermined intervals and detecting this wobble at the time of recording or reproduction, a clock synchronized with the rotation speed can be obtained. Further, by adding a modulation to the wobble, auxiliary information related to the address and the disc is recorded as a physical form in the groove. By detecting the address information recorded in the groove (ADIP: address in pre-groove), information can be recorded in a desired position on the disc.
In the case of a method in which recording is performed only in the groove, for the wobble, the side walls on the both sides of the groove are shaped into the same configuration. The same applies to the case where recording is performed only on the land. In the following, when the groove is the recording area, this will be referred to as groove track, and likewise, when the land is the recording area, this will be referred to as land track. Therefore, in the case of the land-groove method, a land track and a groove track are present. In the land-groove method, the adjoining land track and groove track share the same side wall. For this reason, if an independent address is assigned to each of the land track and the groove track, a section occurs where the configurations of the side walls on both sides of the groove or the land cannot be made the same. That is, since the width of the groove or the land changes in such a section, an adverse effect is observed that the amplitude of the recorded signal is modulated as the track width fluctuates. Moreover, in the pertinent section, an adverse effect that the amplitude of the wobble signal also fluctuates is received at the same time. In the following, these phenomena will be referred to as wobble interference. Although the term wobble interference might refer to a different phenomenon in the pertinent technical field, it is used to mean the above in this specification.
If an independent address is assigned to each of the land track and the groove track as described above, although the occurrence of the wobble interference is unavoidable, the frequency thereof can be reduced. For example, according to the technique disclosed in JP-A-2005-166120 (Patent Document 1), by using Gray code for part of the address information, the frequency of the wobble interference due to the adjoining track can be reduced.
In the following, when it is unnecessary to distinguish between the groove and the land, they will be referred to merely as a track.
According to the technique described in Patent Document 1, as means for making the configurations of the adjoining grooves the same as each other as much as possible in order to reduce the wobble interference, Gray code is used for part of the address information. However, since BPSK (binary phase shift keying) is used as the wobble modulation means, there is a problem in that a high wobble interference occurs in a part where the configurations of the side walls of the adjoining grooves are not the same as each other.
Moreover, according to the technique described in Patent Document 1, it is not performed to protect the ADIP by an error correction code (ECC). This is because even if a systematic code is used, since the code word is different from that of the adjoining track, the wobble configurations of this part are highly likely different from each other and as a result, the effect of reducing the frequency of the wobble interference is lessened or lost. The technique described in Patent Document 1 has a problem in that the reliability of the ADIP information reproduced from the wobble signal is low since the ADIP is not protected by the error correction code. In particular, it has a problem of being extremely vulnerable to an ADIP error due to a disc fault such as a defective wobble configuration and an ADIP error due to dust and dirt adhering to the disc surface.